This application claims the benefit of European Patent Application No. 00125189.1 filed Nov. 20, 2000.
The invention relates to a method for characterizing, by means of a measurement, the appearance of an object, more particularly, to a method for characterizing the contribution of the surface to the appearance of an object and for predicting the object""s surface appearance.
Currently, many products are manufactured from plastic. These products have a particular appearance that depends on how the combination of the object""s color and surface texture is perceived by the human eye. The appearance will in general be different when the sample is rotated relative to illumination and/or observer. This is caused by a number of factors, both internal factors such as the kind of material from which the object is manufactured, manufacturing conditions, the colorants (concentrations and types) used in the material, and surface factors, for example, the surface texture, of the object.
Surface texture is one factor considered when an object is to be manufactured. It may, for instance, be required that the surface""s texture has a leather-like character, such as used in interiors of cars. This surface texture also affects the appearance.
There is are known methods for predicting what the color of an object will be (see: Practical Color Measurement, Anni-Berger-Schunn, Joseph W. Goodman, ed. J. Wiley, New York, 1994) based on the kind of material (often plastics such as polymers) and the colorants used for manufacturing the object.
A leading theory, with corresponding equations, which is used to match color is referred to as Kubelka-Munk. Many improved theories have been derived based on this theory. The Kubelka-Munk theory is based on diffuse illumination, which can be understood as light coming from all or at least many angles simultaneously. The experimental technique to create such an illumination uses a so-called integrating sphere and the data measured on such spectrophotometric equipment is treated using the Kubelka-Munk theory. In a spectrophotometer, the detector is usually placed in this integrating sphere at an angle of zero or 8 degrees relative to the perpendicular of the sample to be measured. For the user of such equipment, there is little practical possibility to change angles of viewing or illumination in this equipment. Therefore, the reflectance values measured in this classical way are considered angle-independent. This is essentially true, although in principle, a redesign of these spectrophotometers with a different viewing angle may lead to a slightly different reflectance value.
The kind of plastic in its natural state and color, as well as the colorants mentioned, serve as a basis in Kubelka-Munk theory for the prediction of an object""s final appearance. However, this prediction is often poorly related to visual perception. For example, the effect of surface characteristics such as gloss on appearance is often not or poorly predicted, and also the effect of changes in illumination and/or viewing angle relative to the surface is not predicted. The integrating sphere used in the Kubelka-Munk (K-M) theory in general also masks the surface effects, for example, the effects of gloss and texture, to a large extent. Thus, neither the K-M theory, as used today, nor its associated measurement is able to predict surface characteristics such as gloss and texture. As a consequence, use of the known art of color prediction in general predicts a color, which is presumed to give a preselected appearance, but in reality more probably results in an appearance totally different than that seen by the human eye. This is particularly true when attempting to match the appearance of different materials.
For example, when a color standard made in polypropylene is used to match a sample to be formulated in a different material, for example ABS (Acrylonitrile Butadiene Styrene copolymer) a predicted formulation using existing color formulation techniques will almost always lead to a visibly unacceptable match. It happens frequently that the measurement of the standard and formulated sample with such equipment indicates that a color difference is small or negligible whereas visible differences are clearly much larger and often unacceptable. Another related problem is that while a visible match can be obtained under one viewing angle, rotation of the sample and reference material may lead to visible appearance differences at some other angles.
More particularly, this means, for instance in the car industry, that it is not easy to manufacture two or more plastic objects having the same appearance such as matching the appearance of one part of the instrument panel with the appearance of another part of the instrument panel made of another material. Thus, an observer notices differences in appearance. In another example, it is not possible to give, for instance, the automotive instrument panel the same appearance as the leather upholstery of a seat. In this case, too, a user of the car notices appearance differences. As mentioned, there are techniques known per se for measuring the color of an object. This technology can also be used to predict what the effects will be of the choice of a particular plastic and colorants (and other additives) on the color of an object to be manufactured. In this way, with existing techniques, an object such as the automotive instrument panel can be manufactured which, as regards its color, corresponds as much as possible with the color of the reference object, such as the leather-upholstered seat mentioned. It has been found, however, that the user still notices differences in appearance.
An object of the invention is to provide a solution to the problems outlined above. In the present invention, directional or collimated light (that is, more or less narrow beam of light which has distinct (although possible somewhat spread) viewing and illumination angle(s)) is used. Viewing angle can be chosen by the user and is a variable that may have significant effect on the result. Measured reflectance data are strongly dependent on these angles (that is, are xe2x80x9cangle dependentxe2x80x9d), and are related to the observed appearance. In contrast, the known systems are angle independent systems or, fixed angle systems, that do not allow to the characterization, measurement or prediction of appearance as observed under a variety of conditions, met in practice.
Accordingly, in a first aspect, the present invention is a method for characterizing the contribution of the surface to the appearance of an object. This method is characterized in that the surface contribution to appearance, or the surface reflection k1(w,h,i), is calculated from a plurality of reflectance values Rm(w,h,i) which are established for at a light frequency w and a plurality of combinations of viewing angle(s) (h), and illumination angle(s) (i). The calculation of k1(w,h,i) from Rm(w,h,i) involves the use of the color determining parameters of the material. Preferably, k1(w,h,i) is calculated at a number of wavelengths, w, covering the visible spectrum, and a plurality (at least two) combinations of viewing and illumination angles in order to have a more or less complete description of the surface contribution to the appearance. Once the surface contribution to the appearance has been determined, the appearance of the sample to the reference material can be subsequently matched by, for example, changing the colorants or other additives.
The invention is based inter alia on the insight that the appearance of the object depends not only on colorant loading and base material, but also on the angle at which the object is viewed by the human eye and the condition of illumination of the object. The effect that the viewing and illumination angles have on the surface and hence the appearance as perceived by the viewer is taken into account. The illumination is preferably directional light although illumination by a combination of directional and diffuse light may be employed.
In this invention, use is made of the formulae in which the surface reflection k1(w) was assumed to be independent of the viewing and illumination angles (h),(i). The formulae are generally based on diffuse illumination that is obtained with an integrating sphere. (See: Practical color Measurement by A Berger-Schunn page 114. Or: Judd, Wyszecki, Color in business, science and Industry, J. Wiley, New York, 1975, p 420-461). In the present invention, however, the formulae known per se (or future formulae still to be developed) are used for determining, for each predetermined viewing angle (h) and predetermined illumination angle (i), what the surface reflection k1(w,h,i) is for these angles. It has been found that in this way these formulae can be used for characterizing the appearance of an object and used for predicting what the appearance of an object will be as well as matching an object""s appearance to a preselected appearance, either an actual reference object whose appearance is desired or a virtual object having a desired appearance. There is more than one color theory. The invention is not limited to an equation of a specific color theory.
In this method, a plurality of (that is, at least two) reflectance values R(w,h,i) for a plurality of sets of (h,i) of viewing and illumination angles wherein each set comprises a viewing angle (h) and an associated illumination angle (i) are measured and a plurality of reflection values k1(w,h,i) corresponding with the respective measured reflectance values R(w,h,i) are calculated. The surface reflections k1(w,h,i) associated with the viewing and illumination angles are calculated from each value of measured reflectance value Rm(w,h,i) associated with these viewing and illumination angles and the value of R1(w).
In another aspect of the present invention, an object""s appearance is predicted from both color determining (xe2x80x9cinternalxe2x80x9d) parameters of the material (which are dependent on the kind of material and the optional colorants from which the object is manufactured) and also the value of the surface reflection k1(w,h,i) for predetermined viewing and illumination angles. Preferably, this method is further characterized in that based on the color determining parameters of the material from which the object is manufactured and on the basis of a plurality of reflection values k1(w,h,i), it is predicted what the appearance of the object will be such as it is observed for the plurality of sets (h,i) of predetermined viewing and illumination angles.
Combining the method according to the invention for characterizing the appearance of an object on the one hand and predicting the appearance of an object on the other hand provides a sound basis for appearance matching.
In such method, the reflectance value of a reference object Rmref(w,h,i) or a number of desired reflectance values at a range of wave lengths associated and/or viewing/illumination angles is determined. This may be established from actual measurements of an object (that is, a real reference object) or a theoretical appearance that is desired (a xe2x80x9cvirtualxe2x80x9d object). Then, a sample object is produced that is selected to have reflectance value or values that approximate the predetermined value or values. This method comprises the steps of:
A. measuring or otherwise setting the reflectance values of a reference object whose appearance is to be matched at a plurality of predetermined viewing (h) and illumination (i) angles;
B. measuring the reflectance value Rmtest(w,h,i) of a test object manufactured from a pre-selected material having a pre-selected amount and type of colorant(s) and/or other additive(s) using a pre-selected method of manufacture;
C. calculating the surface contribution to appearance or the reflection value(s) k1(w,h,i) of the test object from reflectance value(s) Rmtest(w,h,i) associated with the predetermined viewing (h) and under illumination (i) angles;
D. making a sample object predicted to have the reflectance value(s) Rp(w,h,i) which are predicted using the k1(w,h,i) values measured in C. with the desired reflectance value(s) Rmref(w,h,i) of the reference object;
E. comparing the reflectance values measured on the sample object from step D. with that of the reflectance value of the reference object;
F. repeating, as desired, steps B., C., D., or E. using different selection of the amount or type of colorants, other additives or manufacturing process until the surface appearance of the first and second object are acceptable. This choice is made based on the calculated k1(w,h,i) and selected combination of internal factors such as the type and amount of colorants which should approximate the measured reflectance value of the reference object.
In a preferred embodiment, the test object produced in step B. is produced having a black color for determining the surface reflection k1(w,h,i). In that case, the surface reflection has, with respect to the color of the object, a relatively greater influence on the appearance, and can therefore be accurately determined in an easy manner. Using the k1(w,h,i) value measured from the black object, it is then possible to more easily select the internal factors such as colorants to match appearances, both surface appearance and color. In a preferred embodiment, steps B. and C. may be repeated to result in a different surface reflection k1 that leads to a better match of the reflectance values prior to preceding to step D. et seq.
Using the method of the present invention, it is possible to measure and predict the internal (for example, choice of the material, including the type and amount of colorants and/or other additives) and surface factors that contribute to appearance and modify either or both factors to result in the desired appearance. The appearance of the test and reference objects can be matched by changing either or both the surface characteristics or internal factors. It is this combination of surface and internal factors that give an object its appearance and this combination is selected to result in the same appearance even if each factor between the test and references objects are not equivalent.
Color theory, which equations have been developed for diffuse light (that is, assumed to be independent of the viewing and the illumination angles) are, for the purposes of this invention, used with directional light and a plurality of viewing and/or illumination angle(s). Thus, according to the present invention, the appearance (that is, the total of the surface and internal contributions) of a number of objects can be matched essentially over the entire range of viewing angles and wavelengths at which the objects may be viewed. In the prior art methods, it was not practical to match more than color of the objects as measured by diffuse spectrometry and then, the appearance varied between objects as the viewing angles and wavelengths changed.
There are essentially limitless combinations of material, colorants, additives and manufacturing processes that affect appearance. For example, the surface contribution to appearance or the surface reflection k1(w,h,i) may be set based on the choice of material and a surface texture for the object. Alternatively, the surface texture and the kind of material may be fixed and, in this case, the selection of the colorants or other additives is modified to achieve an appearance match.
It is also possible, particularly where the appearance match is not sufficiently close, that the surface texture and/or the type of material will need to be adjusted. In such case, the value of k1(w,h,i) is determined again according to step A and steps B, C, D, E and F may have need to be repeated more than a few times; particularly, where the appearances or the objects being matched are to match at a multitude combinations of viewing and illumination angles.
According to a preferred embodiment of the present invention, the predetermined reflectance value(s) Rmref(w,h,i) is determined on the basis of a reference object. In most cases, this reference object is an existing part; for example, leather seat, that is required to be matched; for example, by the automotive instrument panel. However, the reference object may also be virtual with its reflectance values merely theoretical, that is, set by the desired appearance. By the method of the present invention, an appearance match is obtained between the reference object or existing part and the object to be manufactured.
The present invention has a number of obvious applications, such as matching adjoining parts made from different materials. Thus, two different polymers such as ABS and polypropylene can be made to have the same appearance over the entire range at which such objects are to be viewed. It should also be possible to characterize the surface appearance and to predict and match the appearance of transparent and translucent objects.